In recent years, porous nanoparticles are used in various applications such as adsorption, separation, and catalyst. Porous nanoparticles refers to nanoparticles having a pore of 2-50 nm diameter (mesopore). In particular, porous nanoparticles of titanium oxide are mainly used for white pigment, catalyst support, optical catalyst, reaction catalyst, optical semiconductor, solar battery, etc. since they have unique optical properties, optoelectronic properties, biological properties, sustained releasability, electrical properties, and chemical properties.
Because of their spherical shape, it has become clear that the porous titanium oxide nanoparticles exhibit excellent properties such as excellent stability, excellent dispersibility, high light harvesting characteristics, and easy reuse. Hydrothermal method, sol-gel method, self-aggregation (self-organization) method, and the like are conventionally used as a method for synthesizing spherical porous titanium oxide nanoparticles.
Non-Patent Document 1 describes a method for synthesizing spherical porous titanium oxide nanoparticles used for optical catalyst using the hydrothermal method. Specifically, Non-Patent Document 1 describes a method that Ti(SO4)2, NH4F, and H2O are reacted for six hours at the temperature of 160° C. so as to synthesize spherical porous titanium oxide nanoparticles.
Non-Patent Document 2 describes a method for synthesizing spherical porous titanium oxide nanoparticles used for solar battery using the sol-gel method. Specifically, Non-Patent Document 2 describes a method that Ti(OC4H9)4 and diethylene glycol are stirred in acetone for eight hours, and then centrifuged for 1 hour so as to synthesize spherical porous titanium oxide nanoparticles. Non-Patent Document 3 describes a method for synthesizing spherical porous titanium oxide nanoparticles used in biochemistry (drug delivery) using the self-aggregation (self-organization) method. Specifically, Non-Patent Document 3 describes a method that titanium oxide particles are aggregated, coated with SO2 to form a cluster, and then calcinated and silica-etched so as to synthesize spherical porous titanium oxide nanoparticles. However, there is a problem that these conventional synthesizing methods are very complicated, and thus take a long time for synthesis.
A method for synthesizing spherical porous nanoparticles in supercritical fluid has also been known. Non-Patent Document 4 describes a method for synthesizing spherical porous Fe3O4 nanoparticles in supercritical fluid, and Non-Patent Document 5 describes a method for synthesizing spherical porous TiO2 nanoparticles in supercritical fluid. A method for synthesizing spherical porous titanium oxide nanoparticles using titanium isopropoxide and an organic modifying agent in supercritical fluid is also known. Hexanoic add, hexanal, decylphosphonic add, and the like are known as the organic modifying agent. These methods for synthesizing spherical porous nanoparticles in supercritical fluid use one-pot synthesis and thus have advantages of short reaction time and easy operation. However, in these synthesizing methods, adjustment of the particle diameter and the pore diameter of the spherical porous titanium oxide nanoparticles in accordance with the application thereof has not been easy.